1. Technical Field
The present invention pertains to systems for measuring properties of optical detectors. In particular, the present invention pertains to a system for measuring optical detector linearity by identifying signal portions attributed to detector non-linearity based on a harmonic analysis of measurement signals detected by the detector.
2. Discussion of the Related Art
Various instruments and devices employ optical detectors for operation. The characteristics of the detectors and associated electronics may adversely affect or distort results produced by the instruments or devices. Accordingly, the related art provides several techniques to measure detector characteristics, such as non-linearity. For example, a calibrated radiance target technique utilizes a black body radiator target and a change of target temperature. The photon flux emitted by the black body target is a function of temperature and allows a calculation of the photon energy striking an optical detector. Thus, changing the black body target temperature and measuring the detector response over the optical range of interest results in data that can be plotted and analyzed to determine non-linearity of response.
However, the calibrated radiance target technique suffers from several disadvantages. In particular, this technique is subject to substantial measurement errors due to black body targets reflecting optical radiance originating from the surrounding environment and the uncertainty in the black body target surface temperature. Further, the calibrated radiance target technique typically cannot be relied upon to perform non-linearity characterization to an accuracy better than 0.5% and requires expensive deep well cavity targets with precise temperature monitoring and a test set-up designed to limit environmentally induced errors. Moreover, this measurement technique cannot be performed fast enough to eliminate detector drift errors, while the target temperature is altered. In addition, the calibrated radiance target technique can only characterize linearity in a static DC sense and does not provide linearity characterization as a function of detector operating frequency.
A second method of performing non-linearity characterization involves a dual source progressive profiling technique. This technique uses two radiant sources illuminating an integrating sphere to measure non-linearity of an optical detector. One source or bulb is enabled and a measurement performed. The first bulb is disabled and the second source or bulb is enabled with bias adjusted on the second bulb to attain the same sensor or detector reading as the first bulb. Subsequently, both bulbs are enabled and the detector output should exactly double. This process of switching, comparing and doubling continues in a binary growth fashion until the entire range of interest is covered.
The dual source progressive technique suffers from several disadvantages. In particular, this technique incurs measurement errors due to the sources heating the integrating sphere and altering sphere characteristics. Thus, the dual source progressive technique requires the integrating sphere temperature to be well stabilized. Further, the technique is not fast enough to eliminate detector drift errors in response to the source targets changing temperature and does not perform well at IR wavelengths due to the uncertainty of the integrating sphere background radiance. Moreover, this technique can only characterize non-linearity in a static DC sense and does not provide characterization as a function of detector operating frequency.
A third method representative of the related art uses a harmonic analysis technique combined with an electrically modulated light emitting diode (LED) for measuring non-linearity of an optical detector. The bias current to an LED optical source is modulated in a sinusoidal fashion. The detected optical signal is processed through a spectrum analyzer to detect harmonics generated by the optical detector. The magnitude of each detector generated harmonic allows an assessment of non-linearity.
The LED harmonic analysis technique suffers from several disadvantages. In particular, this technique has limited performance capability due to non-linearity of the modulated optical source which is often greater than the detector undergoing characterization. Further, the LED harmonic analysis technique has a poor signal-to-noise (S/N) ratio to resolve harmonics due to the broadband emission of LED emitters.
A fourth method similar to the modulated LED technique uses harmonic analysis combined with an electrically modulated laser optical source for measuring non-linearity. A laser diode with sinusoidally modulated bias current serves as the optical source. The detected optical signal is processed through a spectrum analyzer to detect harmonics generated by the optical detector. The magnitude of each detector generated harmonic allows an assessment of non-linearity.
The laser harmonic analysis technique suffers from several disadvantages. In particular, this technique has limited performance capability due to non-linearity of the electrically modulated optical source which is often greater than the detector undergoing characterization. Further, the laser diode can only be successfully modulated over a small fraction of its output range, thereby introducing an unwanted optical bias for detector non-linearity characterization and a much reduced signal-to-noise ratio for making the characterization.
The present invention provides several advantages. The present invention employs harmonic analysis of a detected and sampled optical signal in combination with an opto-mechanical modulator of laser energy. The opto-mechanical modulator of laser energy combines with an optical filter to produce a harmonic-free sinusoidally modulated laser source for testing a detector. The opto-mechanical modulator further uses a metrology laser to measure movement of the opto-mechanical mechanism that produces the modulation. The detected optical signal is sampled synchronous to the modulator metrology laser, thereby assuring perfect sinusoidal modulation sampling regardless of the exact speed of the mechanical mirror movement of the modulator. The precision of the sinusoidally modulated optical source significantly exceeds that of the harmonic analyses described above, thereby enabling non-linearity measurements with enhanced accuracies (e.g., 0.01% or better).
The complete non-linearity measurement can be conducted by the present invention in a short time interval (e.g., a small fraction of one second), thereby eliminating measurement drift errors associated with the calibrated radiance target and dual source progressive techniques described above. Since the optical source is modulated at a specific sinusoidal frequency, all errors due to background radiance variation in the calibrated radiance target and dual source progressive techniques described above are also eliminated. The opto-mechanical modulator can be used to change modulation frequency allowing characterization over the detector frequency range which cannot be accomplished by calibrated radiance target and dual source progressive techniques described above.
The present invention employs a monochromatic laser diode rather than a broadband optical source as described above for the LED harmonic analysis technique. This allows a higher S/N ratio in the harmonic analysis and a characterization over a dynamic range one-hundred times greater than the calibrated radiance target, dual source progressive and LED harmonic analysis techniques described above. The use of a monochromatic laser diode further enables the spectrum analysis to differentiate between unmodulated photon flux and several sources of unmodulated background photon flux that lead to errors in the calibrated radiance target, dual source progressive and LED harmonic analysis techniques described above.
The present invention employs an opto-mechanical modulator with sampling synchronous to the modulator mirror movement, thereby providing enhanced performance and modulation over a wider portion of the source range relative to the laser harmonic analysis technique described above. Further, the present invention can produce a wider combination of modulated and unmodulated photon flux test combinations for testing dynamic non-linearity under various static optical flux biases which are not attainable by the calibrated radiance target and dual source progressive techniques described above.
The present invention may be applied to perform precision non-linearity characterization on scientific remote sensing instruments for climate and weather monitoring. However, the present invention may be applied on a broad scope, especially to those applications pertaining to establishing scientific standards for detector linearity and optical test equipment for UV, visible and IR detector characterization. The present invention can improve the accuracy of scientific instruments that must accurately measure incident photon flux.